PROJECT SUMMARY/ABSTRACT Aging is the most dominant risk factor for Alzheimer's disease (AD). Considering the rate the human population is aging, it is imperative to identify means by which to maintain cognitive integrity by protecting against, or even counteracting, the effects of aging. It has become clear that dysfunction in the aged brain is driven by specific alterations at the synaptic level - impairments of which are believed to facilitate later onset of neurodegenerative disease. Thus, identifying molecular mediators of age-related synaptic decline may provide targets to counteract the effects of aging promoting onset of neurodegenerative diseases, including AD. The dynamic post translational modification, O-GlcNAcylation, has emerged as an attractive target for regulating aging-specific synaptic and plasticity-related molecular alterations, as well as neurodegenerative phenotypes. O-GlcNAcylation is regulated by two enzymes O-GlcNAc Transferase (OGT) and O-GlcNAcase (OGA), which catalyze the addition and removal of the dynamic sugar moiety O-GlcNAc to serine and threonine residues of proteins, respectively In the adult mammalian brain, total OGT (ncOGT and sOGT isoforms) and O-GlcNAc are highly expressed in the hippocampus, and mediate various aspects of synaptic plasticity underlying cognitive function. During aging levels of the most active OGT isoform (ncOGT) decrease in the mouse hippocampus in a timeframe consistent with age-related cognitive decline22, and preliminary data demonstrate decreased O- GlcNAc levels in the hippocampus with age. Interestingly, O-GlcNAc levels in the brains of AD patients are also decreased compared to healthy controls32, and preliminary data demonstrate that total OGT expression decreases in the hippocampus of hAPP transgenic mice. While there is much speculation about the role of O- GlcNAc within the context of neurodegeneration, its role in contributing to both physiological brain aging and AD- related pathology has not yet been thoroughly investigated. Therefore, the purpose of the proposed study is to define the role of O-GlcNAcylation in regulating aging-associated versus neurodegenerative phenotypes in the adult hippocampus. Specifically, we hypothesize that isoform specific loss of OGT promotes aging-related synaptic impairments versus neurodegenerative phenotypes, while restoring O-GlcNAcylation rejuvenates the aged brain and ameliorates AD-related neurodegeneration. We will test this theory with three Specific Aims: 1. Distinguish the role of ncOGT and total OGT in regulating aging-associated versus neurodegenerative phenotypes in the adult hippocampus. 2. Examine synaptic changes in O-GlcNAcylated proteins in the adult hippocampus during normal aging and AD-like pathology. 3. Investigate the effects of increasing hippocampal O-GlcNAc levels in normal aged mice and a mouse model of AD. Ultimately, this study will identify post translation modifications as potential molecular therapeutic targets to counteract brain dysfunction in the elderly and consequently combat susceptibility for dementia-related neurodegenerative diseases, including AD.